Making a call to a .NET 4 library from 3.5

No. You'll need to target your project to run in .NET 4, and to use the CLR v4, in order to use .NET 4 assemblies.

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On a different note - there is no real advantage here. Just change your application to target .NET 4. If you are going to add a dependency on the .NET 4 framework assemblies, you might as well just target .NET 4 in the first place...

Here's a kind of Rube Goldberg-esque solution which involves wrapping your .NET 4 DLL methods in COM, and then calling that COM wrapper from .NET 3.5:

https://www.codeproject.com/Articles/204406/How-To-Use-a-NET-Based-DLL-From-NET-Based-Appl